The present invention relates to a method for compatibly increasing resolution in a color television system based on a procedure in which, at the transmitter, part of the signal is converted in frequency by offset modulation and, at the receiver, that signal part is reconverted by offset demodulation, with a reference signal for the offset demodulation being transmitted on an empty line during the vertical blanking interval.
For a compatible increase in detail resolution in color television transmissions, the periodical "Fernseh- und Kinotechnik" [Television and Cinema Technology], Volume 34, No. 2/1980, pages 41-48, describes a method of so-called offset sampling with the use of planar pre- and post-filtering. Similar sampling methods, so-called sub-Nyquist sampling methods, are described in "A single Channel HDTV Broadcast System--the MUSE--", NHK Laboratories Note, Ser. No. 304, September, 1984.
In the above-mentioned procedure, the signal is transmitted via a limited bandwidth channel, with part of the signal, within a partial frequency range, which produces the increase in resolution, not being transmitted in its original spectral position but being changed, by the offset sampling or offset modulation, to a carrier frequency position, i.e. a so-called offset position. For example, a signal component which can no longer be transmitted by means of a conventional television channel of a bandwidth of 5 MHz is converted by the offset modulation to a frequency position within the channel which can then be transmitted, together with the actual baseband video signal of 0 to 5 MHz, over the conventional television channel.
In the prior art method, the sampling modulation frequency, i.e. the frequency of a signal producing the offset modulation, lies outside the bandwidth of the transmission channel. At the playback end, offset demodulation must return the signal, that has been transposed into the offset position, back to its original frequency position so that it can produce the increase in resolution. The reconstruction of this signal component in the original spectral position requires synchronous post-sampling/demodulation at the receiver. The spectral components transmitted at their base position and the spectral components returned to the original position from the carrier frequency position by offset demodulation are combined in the prior art method by means of a Nyquist filter.
In a compatible embodiment for a standard receiver, the additional signal information carried in the channel produces adverse influences which, depending on the manner of distribution of the additional information over two or more fields in the standard line interlace system, express themselves in the form of flicker interference at different frequencies. If the luminance and chrominance information are not transmitted without crosstalk (PAL, NTSC, SECAM) the additional signal information produces further crosstalk problems. To improve compatibility, DE-OS [Federal Republic of Germany Laid-open Application] No. 3,344,524 and counterpart U.S. application Ser. No. 06/680,292 now U.S. Pat. No. 4,723,157, disclose a modulation method (offset modulation) equivalent to offset sampling which employs a reduction in amplitude of the additional signal information at the transmitting end.
A synchronization method which ensures synchronous postsampling or offset demodulation at the receiving end is disclosed in DE-OS No. 3,414,271, and counterpart U.S. Pat. No. 4,661,839]. This synchronization method operates with a phase reference signal which at one-half the sampling/modulation frequency lies at the 6 dB point of the Nyquist filter.
With an application of the modulation method according to DE-OS No. 3,344,524, it is no longer obligatory for communications technology reasons to equip the entire system with a Nyquist channel. Therefore, the spectrum of the total information can be subdivided according to other considerations within the bandwidth of the transmission channel.
To further improve compatibility if the offset modulation method is employed in the PAL system, DE-OS No. 3,435,264, and counterpart U.S. Pat. No. 4,670,773 propose filtering the additional signal information through a lowpass filter. This reduces the region of overlap between the carrier frequency chrominance information and the additional luminance information (increase in resolution) so that the compatible reception is no longer adversely influenced as a result of cross-color interference due to the offset modulation.
Because of the lowpass filtering, the spectral range of the total information is no longer subdivided symmetrically as for Nyquist filtering; rather the frequency band is divided up asymmetrically. Thus the phase sensitive point of the spectral addition can no longer be represented by a reference signal at one-half the modulation frequency.